The removal of a sorbable component from a gas or vapor stream by flowing such stream through a body of adsorbent material is a fundamental engineering practice. One type of sorbable components which are desirable to remove from a gas stream is volatile organic compounds VOCs.
VOCs are formed in large quantities but at relatively low concentrations from gas turbines, cogeneration plants, petrochemical plants, and in many industrial processes where waste gases contain such materials as vapors of various solvents, inks, paints, and so forth. VOCs contain not only hydrocarbons--saturated, unsaturated, and aromatic--but also contain oxygenated materials such as alcohols, esters, ethers, aldehydes, and acids, nitrogen containing compounds (principally amines), sulfur containing materials (mercaptans and thioethers) and halogen-containing materials, especially chlorine-substituted hydrocarbons but also organic fluorides and bromides. The presence of these VOCs in the gas stream can present a health risk or cause the gas stream to have an unpleasant odor.
The widespread use of solvents in industrial applications has resulted in increased emissions of VOCs into the atmosphere, giving rise to environmental concerns and prompting stricter legislative controls on such emissions. As a consequence, manufacturers of pharmaceuticals, coated products, textiles, and polymer composites and foams, as well as hydrocarbon producers and distributors, face a dilemma in removing VOCs from process gas streams in that, owing to rising energy prices, recovery costs are very often higher than the value of the VOCs recovered, even in light of rising solvent prices. This dilemma has led to inquiries into more profitable methods of removing condensible organic vapors from process gas streams. A recent article titled, "Select the Best VOC Control Strategy," by Edward N. Ruddy and Leigh Ann Carroll which appeared in the July 1993 issue of "Chemical Engineering Progress," page 28-35 summarized current emission control options of thermal oxidation, catalytic oxidation, condensation, carbon adsorption and absorption. In the article, Ruddy and Carroll state that VOCs are among the most common pollutants emitted by the chemical process industries and the reduction of VOCs is, therefore, a major environmental activity.
Conventional adsorption systems for solvent recovery from humid air typically are operated until the solvent concentration in the outlet gas stream reaches a detectable preset breakthrough level whereupon the gas flow to the adsorber is stopped. The adsorbent bed then contains solvent, other condensible organic contaminants, and some amount of water which depends on the inlet relative humidity of the solvent laden gas stream. At this point, present-day techniques involve the introduction of a hot inert gas or steam, either saturated or superheated, which displaces the solvent from the adsorbent to produce a solvent/water mixture upon condensation. Typically two adsorber beds are used, where one is adsorbing while the other bed undergoes regeneration. More recent technology for regenerating and recovering solvent from adsorbent beds involves the use of inert gases (though for some solvents, air also can be used) and low temperature condensation of the solvent from the regenerating gas.
The removal of volatile organic compounds (VOC) from air by adsorption is most often accomplished by thermal swing adsorption (TSA). Air streams needing treatment can be found in most chemical and manufacturing plants, especially those using solvents. At concentration levels from 500 to 15,000 ppm, recovery of VOCs from air is economically justified. Steam is used to thermally regenerate activated carbon adsorbent. Concentrations above 15,000 ppm are typically in the explosive range and require the use of a hot inert gas rather than air for regeneration. Below about 500 ppm, recovery is not economically justifiable, but environmental concerns often dictate adsorptive recovery followed by destruction. Activated carbon is the traditional adsorbent for these applications, which represent the second largest use for gas phase carbons. U.S. Pat. No. 4,421,532 discloses a process for the recovery of VOCs from industrial waste gases by thermal swing adsorption including the use of hot inert gases circulating in a closed cycle to desorb the VOCs.
Pressure swing adsorption (PSA) processes provide an efficient and economical means for separating a multi-component gas stream containing at least two gases having different adsorption characteristics. The more strongly adsorbed gas can be an impurity which is removed from the less strongly adsorbed gas which is taken off as product, or, the more strongly adsorbed gas can be the desired product which is separated from the less strongly adsorbed gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feedstream to product a purified (99+%) hydrogen stream for a hydrocracking or other catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more strongly adsorbed gases, such as ethylene, from a feedstream to produce an ethylene-rich product.
In PSA processes, a multi component gas is typically passed to at least one of a plurality of adsorption zones at an elevated pressure effective to adsorb at least one component, i.e. the more strongly adsorbed components, while at least one other component passes through, i.e. the less strongly adsorbed components. At a defined time, the passing of feedstream to the adsorber is terminated and the adsorption zone is depressurized by one or more cocurrent depressurization steps wherein the pressure is reduced to a defined level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. Then, the adsorption zone is depressurized by a countercurrent depressurization step wherein the pressure in the adsorption zone is further reduced by withdrawing desorbed gas countercurrently to the direction of the feedstream. Finally, the adsorption zone is purged and repressurized. Such PSA processing is disclosed in U.S. Pat. No. 3,430,418, issued to Wagner, U.S. Pat. No. 3,564,816, issued to Batta, and in U.S. Pat. No. 3,986,849, issued to Fuderer et al., wherein cycles based on the use of multi-bed systems are described in detail. As is generally known and described in these patents, the contents of which are incorporated herein by reference as if set out in full, the PSA process is generally carried out in a sequential processing cycle that includes each bed of the PSA system. A VSA process employs a similar sequential processing cycle with the countercurrent depressurization step assisted by a vacuum pump or similar device to evacuate the adsorption zone to reach desorption conditions.
As noted above the more strongly adsorbed components, i.e., the adsorbate, are removed from the adsorber bed by countercurrently depressurizing the adsorber bed to a desorption pressure. In general, lower desorption pressures are preferred in order to provide more complete removal of the adsorbate during the desorption step. U.S. Pat. No. 4,338,101 discloses a process for the recovery of hydrocarbons from inert gas and hydrocarbon mixtures which includes the steps of adsorbing the hydrocarbons in a bed of solid adsorbent and desorbing the hydrocarbons from the bed of adsorbent by evacuating the bed using a vacuum pump. U.S. Pat. Nos. 5,154,735 and 5,229,089 disclose similar processes for recovering hydrocarbons from a hydrocarbon-air mixture employing further vacuum pumping steps to enhance desorption from the solid adsorbents.
U.S. Pat. No. 4,842,621 discloses a process for separating a condensible gas from a non-condensible gas in a vacuum swing adsorption scheme which includes a supercharging step in which a stream relatively rich in the condensible gas is passed through the adsorbent bed following the adsorption step to reduce the amount of condensible gas recycled to the bed in the adsorption step. The supercharging step occurs at a pressure which is higher than the adsorption pressure.
When the impurities are highly volatile or explosive in mixtures with air or other gases, approaches must be employed which avoid the potential safety hazard of fire or explosion should the recovered VOC stream approach a composition in its detonation region. Processes are sought which permit the safe concentration and economic elimination of VOCs from waste gas streams without the further dilution of the waste gas stream with inert gases which raise the capital and operating costs.